F. SHEHATA ET AL.
32
values are increased as numbers of ECAP passes are in-
creased. After the first pass, the hardness has increased to
1.5 times the as cast composites. The maximum hardness
obtained after 8 passes for both composites showed al-
most 2.9 to 3.5 times higher than the corresponding val-
ues of the as cast MMC composites.
It was found that 0.2% proof stress and hardness of
both the composites after ECAP are impressively higher
than as cast pure aluminium used in this work. The flow
stress of pure aluminium after eight ECAP passes is re-
ported to be 132 MPa [21] compared to 233 MPa in this
work which is considerably higher than the repotted
value. This may indicate that the SiC is very effective in
increasing strength or hardness when ECAP is applied.
As shown Al-10% SiC shows higher strength than Al-5%
SiC composite due to presence of higher amount of SiC
particles. Dislocations are generated due to mismatch in
thermal expansion coefficient between the matrix and the
reinforcement. As a result, the matrix of composites con-
tains higher dislocation. Higher the volume fraction of
reinforcement higher will be the dislocation density. This
leads to higher hardness and strength with increase in
SiC content.
4. Conclusions
1) Commercial purity aluminum matrix with SiC rein-
forcement can be successfully fabricated using conven-
tional low cost method of stir casting.
2) The distribution of silicon carbide particles has
shown an aggregate structure in as cast composites. The
stir cast leads to breaking down most of the SiC aggregates.
3) Composite reinforced with 10% SiC showed greater
agglomerations and porosities compared to 5% SiC in as
cast condition.
4) ECAP techniques resulted in structural refinement
and SiC particles have greatly reduced from 50 µm to 5
µm in Al-5% SiC and 3 µm in Al-10% SiC after the first
ECAP pass.
5) The as cast AlSiC composites indicated porosities up
to 3.6%. After eight ECAP passes, porosity was reduced
1.1 and 1.31% for Al-5% SiC and Al-10% SiC respectively.
6) After the first ECAP pass, yield strength has almost
twice its value in the as cast composites. The maximum
yield of 245 MPa obtained after 8 passes is almost four
times the corresponding values of the as cast MMC com-
posites.
7) After the first ECAP pass, hardness has almost 1.5
times its value in the as cast composites. The maximum
hardness of 71 HRB obtained after 8 passes is almost 3.5
times the corresponding values of the as cast MMC
composites.
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